(1) Field of the Invention
The present invention relates to a filler for a heat-sensitive recording paper. More particularly, the present invention relates to a filler for a heat-sensitive recording paper which comprises finely divided amorphous silica having novel characteristics. Furthermore, the present invention relates to a heat-sensitive recording paper comprising this filler.
(2) Description of the Prior Art
A heat-sensitive recording paper comprising a support such as paper and a recording layer formed thereon, which comprises a dispersion of a coloring agent such as a leuco dye and a color developer capable of forming a color on contact with the coloring agent in the hot state, such as a phenol, in a binder has been widely used for facsimile, printers, data communication, computer terminals, measuring devices, passometers, copying machines and the like while using a thermal head, a hot pen, an infrared ray lamp, a laser or the like as a heat source.
A heat-sensitive recording paper of this type is defective in that when recording is carried out by bringing a recording layer into contact with a recording head or the like, the components contained in the recording layer are fused and adhere to the recording head or the like to cause such troubles as scum adhesion and sticking.
Various fillers have been incorporated into recording layers so as to eliminate this disadvantage. Namely, it has been known from old that calcium carbonate, kaolin, talc, alumina and titanium dioxide are incorporated. Recently, incorporation of a hydrous aluminum silicate mineral (Japanese Patent Application Laid-Open Specification No. 72992/81), amorphous synthetic aluminum silicate (Japanese Patent Publication No. 19035/82), wollastonite or calcium silicate (Japanese Patent Application Laid-Open Specification No. 41995/82), an alkaline earth metal salt (Japanese Patent Application Laid-Open Specification No. 80095/82) and aluminum hydroxide (Japanese Patent Application Laid-Open Specification No. 14093/82) has been proposed.
It seems curious that amorphous silica as a filler is not included in these proposals. It is considered that the reason is that amorphous silica generally has a surface activity and provides an active surface for the reaction between a leuco dye and a phenol and this reaction is hence promoted to cause so-called background coloration.
Amorphous silica by the wet method and amorphous silica by the gas phase method (dry method) are known as finely divided amorphous silica, and they are applied to various uses according to the characteristics thereof. Silica by the wet method is prepared by neutralizing an aqueous solution of an alkali metal silicate such as so-called water glass with a mineral acid or the like. According to this method, a sol of silicic acid is once formed and sol particles are agglomerated to form a gel-like precipitate. Therefore, finely divided amorphous silica has a primary particle size of 10 to 20 millimicrons, but these fine particles tend to agglomerate to form secondary particles having a relatively large particle size. This silica by the wet method is relatively porous and has a large surface activity, and the specific surface area is larger than 200 m.sup.2 /g and is as high as 800 m.sup.2 /g in some cases.
Silica by the gas phase method is prepared by hydrolyzing silicon tetrachloride in an oxyhydrogen flame. The primary particle size of this silica is in the range of from 10 to 20 millimicrons and the surface activity is considerably small, but generally, the specific surface area is larger than 100 m.sup.2 /g.
When silica such as mentioned above is used as a filler for a heat-sensitive recording paper, there are many problems to be solved. The first problem is a problem of the background coloration (background contamination or background fogging) of the recording layer. In the case of silica having a relatively large activity, such as silica by the wet method, the recording layer is colored in an inherent hue before recording and a clear image cannot be obtained. After recording, the background is colored during storage and the storability or life of a print is degraded.
The second problem resides in that most of amorphous silicas have an abrasive property and therefore, the relative movement between the recording paper and the recording head or pen, that is, scanning, is inhibited and the recording head or the recording layer per se is readily worn away.
The third problem resides in that in order to prevent the adhesion of scum, the oil absorption, that is, the bulk, of amorphous silica should be considerably large and in order to prevent the background coloration, the surface activity should be controlled to a level as low as possible, and it is difficult to simultaneously satisfy these two contradictory requirements.